Abstract: The carbon monoxide adduct of Fe complex of meso-mono[o-(4-dietheyla-mino) butyramidophenyl] triphenylporphyrin (tailed porphyrin, meso-MDBPTPP) is reported. The equilibrium constant of the adduct was measured by spectrophoto-metric method and the COvibration was obtained by using differential spectrum technique. Experimental results demonstrate that the fifth ligand affects the equilibrium constant dramatically.